1. Field of the Invention
This invention relates to an automotive vehicle power-steering system and method, and more particularly to an automotive vehicle power-steering system comprising a hydraulic or electromotive power-assist actuator for controlling a manual steering force.
2. Description of the Related Art
Both hydraulic and electromotive automotive vehicle power-steering systems are well known. Japanese published unexamined utility model application No. 56-136855 discloses an example of a hydraulic power-steering system for an automotive vehicle. This system comprises an oil pump driven by an engine, a reservoir, a control valve, a power-assist actuator actuating steerable road wheels, an electrical control unit for the control valve, and connecting hydraulic lines. The control valve includes a spool and two solenoids electrically connected to the control unit. The control unit receives electrical signals T, .omega., .omega. and V and provides the solenoids with controlled electrical currents in response thereto. The signal T represents a manual steering force applied to a steering wheel. The signal .omega. represents the angular deviation of the steering wheel from its neutral position. The signal .omega. represents the angular velocity of the steering wheel. The signal V represents vehicle speed.
In this known system, when steering torque drops to zero, so that the steering wheel tends to return to its neutral position, e.g. after the automotive vehicle exits a corner, the control valve tends to occupy the neutral position at which no oil pressure is applied to the power cylinder. After turning a corner, the steerable road wheels tend to return to a straight-ahead position due to wheel alignment and at the same time, the steering wheel also tends to return to its neutral position. However, since the frictional forces developed in moving parts of the power-assist actuator and of the steering linkage act against the return of the steering wheel to its neutral position, the driver must manually return the steering wheel to its neutral position.
When the steering force applied to the steering wheel is released, the steering system shifts toward its neutral position due to the centering force treated by wheel alignment. However, neutralization of the steering system ends to delay due to resistance caused by friction in the movement of the actuator and/or the steering linkage. This requires a manual neutralizing operation to degrade the drivability of the vehicle.